With Pandora's share price sagging, with big spending on acquisitions like TicketFly, and without a clear plan to growing the core internet radio business, Pandora "may be pursuing a costly and uncertain business plan," writes Meister.

He urges Pandora to hire an outside bank to explore a "value maximization process" that could include a sale.

Here's the important part:

We have become increasingly concerned that the company may be pursuing a costly and uncertain business plan, without a thorough evaluation of all shareholder value-maximizing alternatives. For the reasons set forth below, we urge the company to immediately engage an independent investment bank with a fresh perspective and without any prior history of advising the company to advise on a value maximization process - including the execution of a sales process - and to evaluate the results against other options including the risk-adjusted value of continuing to operate on a standalone basis.

Pandora is on the cusp of realizing an extraordinary vision: fundamentally changing the way listeners discover and enjoy music, and the way artists build and sustain their careers. Pandora has a profitable core business, combined with a strong balance sheet. We are confidently investing to fully capture the massive opportunity ahead of us.

Our management team is in constant dialogue with shareholders about our business strategy and committed to delivering results and long-term value.